For example, in this classic shadow illusion by Edward H. Adelson, A and B are the exact same shade of grey:

 

Here’s a minimalist illustration by Wikipedia user Dodek. The grey bar across the center is actually one constant color:

 

In this image from BrainDen, the surface colors of A and B are the same. To test it out, just use your finger to cover the middle of the drawing, where the two squares meet.

 

In this illusion by Barton L. Anderson and Jonathan Winawer, the black and white chess pieces are the same color:

 

If you want a dog of a different color, just set it against a different background (via BrainDen):

 

There are actually only two colors in this image — red and green (sorry, color blind people). Also via BrainDen.

 

The blue and yellow border around this image by Jochen Burghardt creates the illusion that it is pale yellow, instead of white:

 

Contrasting colors can even give you the illusion of motion, as in this trippy graphic by Paul Nasca:

 

The same principle is at work here, in Akiyoshi Kitaoka’s “autumn color swap.” If you move the page up and down, the inset square will appear to move.

 

If you stare at the center of this illusion by Jeremy Hinton, you will eventually see a revolving green circle. When the lilac disappears, the adaptation of rods and cones in the retina leaves a green afterimage.

 

Or, as in Pinna’s illusory intertwining effect, colors can give the illusion that circles are intertwining (they are actually concentric).

 

But probably the best illusion on the subject of the dress is by Randall Munroe of Xkcd, who immortalized the debate in an optical illusion cartoon form.